feat(app): LoginShell PATH repair and dependency-free include_identity!

[BumpyClock]

Two follow-ups on the AppShell platform stack. Stacks on #8 (base: feat/app-platform, not main); review/merge #8 first.

1. Real EnvironmentPolicy::LoginShell (tsq-5)

LoginShell previously logged a warning and behaved as Inherit. It now performs the macOS/Linux GUI-launch PATH repair that agent-term needs before it can migrate.

• Repair runs in Preflight via the fix-path-env crate (Tauri's login-shell PATH fixer, Apache-2.0 OR MIT, the same crate + rev agent-term already uses). Pulled in only on macOS/Linux; Windows is a documented no-op (GUI processes inherit PATH from the launching context).
• Soundness precondition made explicit. The repair copies the login shell's env into the process via std::env::set_var, which is UB on Unix under concurrent env access. The variant's rustdoc places the obligation on the caller: select LoginShell only from a single-threaded main() before any thread spawns. This mirrors why the deleted Custom(vars) variant was rejected — accepted here only because it's a single vetted repair, not an open-ended env hook. The no-Custom comment block is updated to reference it.
• A failed repair is logged + falls back to Inherit, never a startup abort.
• Tests cover the policy plumbing (default is Inherit, setter records LoginShell, Inherit is a no-op); actual PATH mutation is not asserted in CI.

2. include_identity! needs no direct manifest dependency (tsq-6)

The generated identity source hardcoded ::gpui_component_manifest::schema::IdentityRef, an absolute path that forced every consuming app onto a runtime dependency on gpui-component-manifest (plus a cargo-machete ignore) even when it reached the macro through gpui-component-app's re-export.

• The generated source now names schema types unqualified. include_identity! expands to a private module that imports them with use $crate::schema::{…} before include!()-ing the file. $crate resolves to the defining crate by def-id, so it works transitively through a re-export.
• Apps calling gpui_component_app::include_identity!() now need gpui-component-manifest only as a [build-dependencies] entry (for emit_identity() in build.rs).
• Both example apps drop the direct runtime dep and the machete ignore. The downstream fixture now reaches the macro through a small local re-export shim, proving the dependency-free path in a hermetic test (no gpui compile). A build.rs unit test locks the "no absolute manifest path" contract.

Gates

• cargo test -p gpui-component-manifest -p gpui-component-app — pass (incl. downstream fixture + new env/macro tests)
• app_shell + app_shell_tray build and --smoke exit 0
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --check — clean

Note: cargo machete reports two dependencies (gpui-component-assets in crates/app, smol in examples/system_monitor) — both pre-existing on feat/app-platform, untouched here. This PR introduces no new machete findings.

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